Jama Aden, who is Somali, was held after police raided his hotel room in Sabadell, north of Barcelona.
The coach and an unnamed Moroccan physiotherapist who worked with him have been charged with administering banned substances to athletes.
Mr Aden coaches Ethiopian star Genzebe Dibaba. Neither has commented yet.
Dibaba, 25, is female world athlete of the year and a hot favourite to win gold at the Rio de Janeiro Olympics later this year.
Other athletes who have been coached by Mr Aden include two-time world indoor 800m champion Abubaker Kaki Khamis and 2008 Beijing Olympics 800m silver medallist Ismael Ahmed Ismael, both from Sudan, and 2012 London Olympics 1,500m champion Taoufik Makhloufi of Algeria.
The Spanish anti-doping agency, AEPSAD, said Spanish reports that blood-boosting drug EPO had been found in the physio's hotel room were correct.
It also said it had drug-tested more than 20 athletes at the hotel, in conjunction with governing body the International Association of Athletics Federations (IAAF).
The athletes have not been named and none has yet commented on the raid.(BBC)